Titanium dioxide base pigment particles are often incorporated into many products such as paint, plastic, paper, laminates or other coating systems. This pigment imparts desirable properties such as brightness, opacity, light scattering, and light stability.
Upon prolonged exposure to light, many titanium dioxide pigment products will degrade and the product's color will fade over time. UV radiation from light is particularly damaging to titanium dioxide pigment products. The ability of titanium dioxide to resist degradation when exposed to light is known as light stability and is partially due to the pigment's ability to scatter light.
It is known that light scattering ability of a pigment does not improve above a certain pigment concentration due to an effect known as crowding. The crowding effect severely limits, not only light scattering ability of a pigment, but also, light stability, brightness and opacity. Typically, at high pigment concentrations, when titanium dioxide pigment particles are close to each other, there is an overlap of light scattering causing a severe reduction in the pigment's overall light scattering efficiency and light stability.
There has been considerable research involving ways to reduce crowding and improving light stability. In paper products, keeping the titanium dioxide pigment particles spaced about 0.1 mm or more apart from each other can reduce the crowding effect. One way to keep the pigment appropriately spaced is to add a spacer to the pigment. This may improve light scattering ability and stability as well as brightness and opacity of the pigment.
Spacers typically cost less than the titanium dioxide base pigment and therefore reduce the cost to make the titanium dioxide product. Some examples of spacers used with titanium dioxide products include calcium carbonate, clay, silica, alumina, and other metal oxides.
In general, the size, charge and the resulting flocculation behavior when the spacer is incorporated into the pigment are also important for light scattering and stability. There have been attempts to blend or mix titanium dioxide pigment particles with spacers resulting in spaced pigment products. However, blending or mixing may reduce light scattering ability and light stability of the pigment product due to unwanted agglomeration. For example, when clay is mechanically mixed or blended with titanium dioxide, it is difficult to control the distribution of clay spacer particles throughout the mixture uniformly because the clay particles do not attach effectively to the surfaces of the titanium dioxide particle. The clay particles agglomerate, which reduces light scattering and stability of the pigment product.
Direct in situ precipitation of the spacer on the titanium dioxide pigment is another method to attach the spacer to the base pigment. This method involves precipitating the spacer onto the pigment. However, achieving the proper size and shape of the spacer to produce a titanium dioxide pigment product with improved light scattering ability and stability remains a problem in the art.
There remains a need for titanium dioxide pigments with improved light scattering ability and stability for use in products such as paints, paper, laminates and plastics. Methods and compositions for making spaced titanium dioxide pigments that have minimal agglomeration and improved light scattering ability and stability are needed.